1. Field of the Invention
The present invention relates to a method and apparatus for handling MBMS dynamic scheduling information, and more particularly, to a method and apparatus for handling MBMS dynamic scheduling information in a network terminal of a wireless communication system, so as to enhance MBMS scheduling efficiency.
2. Description of the Prior Art
To enhance multimedia performance of the 3G mobile telecommunications system, the 3rd Generation Partnership Project (3GPP) introduces a Multimedia Broadcast Multicast Service (MBMS), which is a point-to-multipoint bearer service established on an existing network architecture of the Universal Mobile Telecommunications System (UMTS). MBMS allows a single source terminal to simultaneously transmit data to multiple user equipments (UEs) via Internet Protocol (IP) packets.
However, as the multimedia performance of mobile devices advances, consumers are more interested to have multimedia or mobile TV services via the mobile devices. In order to meet such requirement, the 3GPP introduces an enhanced MBMS (eMBMS) in a specification of long term evolution (LTE) Release-9, to support high quality streaming multimedia and real-time MBMS services.
For improving performance of MBMS service, eMBMS introduces a single frequency network (SFN) operation for MBMS transmission, i.e. MBMS Single Frequency Network (MBSFN), to reduce service interruption due to frequency switching during transmissions.
In addition, eMBMS only defines two logical channels to support point-to-multipoint (p-t-m) downlink transmission: Multicast Control Channel (MCCH) and Multicast Traffic Channel (MTCH). MCCH is utilized for transmitting control messages of all MBMS services in an MBSFN, and MTCH is utilized for transmitting session data of an MBMS service. Both MCCH and MTCH are mapped to a transmission channel newly defined by eMBMS, i.e. Multicast Channel (MCH).
In general, an MBSFN has an MCCH. However, when an enhanced Node B (eNB) is simultaneously covered by multiple MBSFN areas, the eNB may have multiple MCCHs. Besides, since an MBSFN area can simultaneously support multiple MBMS services, and different MBMS services may have different requirements, such as Quality of Service (QoS), Block Error Rate (BLER), according to different characteristics, an MBSFN area may have multiple MCHs. Different MCHs suffice requirements of different MBMS services by applying different modulation and encoding schemes. MCCH is mainly responsible for providing these MCHs with the following control parameters:                (1) Modulation and Coding Scheme (MCS): illustrating encoding and modulation scheme of each MCH.        (2) MBMS service list: listing MBMS services currently provided by each MCH, and including MBMS service ID and session ID, and further including corresponding Logical channel ID (LCID) of MBMS services. Noticeably, MBMS service list only provides session information of MBMS services currently processed. In other words, once a session of one of the MBMS services is ended, a network terminal removes session information of the MBMS service from the MBMS service list, to notify a UE related to the MBMS service that the MBMS service session has been ended.        (3) MCH Sub-frame Allocation Pattern (MSAP): defining a pattern of sub-frames used by each MCH in a scheduling period. A set of these sub-frames used by one MCH is called an MSAP occasion. In each MSAP occasion, a network terminal can apply multiplexing of MTCHs and MCCH corresponding to different MBMS services to be transmitted on the MCH. A transmission order of MTCHs can be determined according to an order of the MBMS service list.        
Since an MCH may simultaneously provide multiple MBMS services, while an MBMS UE may be only interested in one of the services, the current specification further provides a Dynamic Scheduling Information (DSI) for an MCH, to indicate the sub-frames used by each MTCH in an MSAP occasion. The following principles are used for the Dynamic scheduling information:                (1) The dynamic scheduling information is used without considering whether multiple services are multiplexed onto an MCH to transmit or not.        (2) The dynamic scheduling information is generated by the eNB, and is transmitted at the beginning of each MSAP occasion, i.e. a first sub-frame.        (3) The dynamic scheduling information allows a UE to determine which sub-frames are used by each MTCH.        (4) How a dynamic scheduling information is carried is for further study, e.g. the dynamic scheduling information can be carried in a MAC Control Element or transmitted via a separate logical channel, such as a Multicast Scheduling Channel (MSCH).        (5) A mapping relation between an MTCH and corresponding sub-frames is determined according to indexes of sub-frames belonging to each MSAP occasion.        
For the current specification, 3GPP change request R2-093093 proposes to use an MBMS scheduling info MAC control element for carrying the dynamic scheduling information. In an MBMS scheduling info MAC control element, each MBMS service contains one ending point index (EPI) field for each MBMS service, for indicating an index of an ending sub-frame for each MBMS service in the current scheduling period.
Please refer to FIG. 6, which is a schematic diagram of an MBMS scheduling info MAC control element. As shown in FIG. 6, a size of each EPI fields is 8 bits. An amount of the EPI fields is equal to that of services provided in the MBSFN area. A mapping relation between the EPI fields and the provided services is one-to-one, and an order of the EPI fields is the same with an order of appearance of the services in the MBMS service list. Therefore, a UE can refer to the order of appearance of these services in the MBMS service list, to derive each EPI representing an index of an ending sub-frame of which MBMS service. Noticeably, it is assumed that each MBMS service uses continuous sub-frames in a scheduling period when MBMS scheduling info MAC control element is defined.
As can be seen from the above, when a particular MBMS service session is ended, the network terminal removes information of the MBMS service from the MBMS service list. However, the MBMS service list is transmitted on an MCCH, and data is transmitted on an MCCH periodically, whereby data repeatedly transmitted within one period is unchanged. Therefore, the network terminal can only update data in a next period. Before the UE receives a new MBMS service list, the UE still refers to the original service list to receive MBMS service on the MCH. Under such a situation, not all MBMS services have data for transmission in the scheduling period. Therefore, how to indicate those MBMS sessions not transmitted in the scheduling period, such that a mapped relation between the EPI fields and services provided on the MCH is correct, is a problem the present invention intends to solve.